High temperature addition polyimides are currently being used as adhesives to bond composite materials and metals such as titanium on advanced air- and spacecraft. These thermoset adhesives undergo cure by an addition reaction involving unsaturated end groups which causes them to be insoluble and highly crosslinked. Though addition-type polyimides can be used for long terms at elevated temperatures of 232.degree. C. (450.degree. F.) and above, their utility is limited because of their brittleness. A method for toughening these polymers is therefore needed to provide the properties of increased peel strength and improved resistance to adhesive fracture.
Several years ago, a particular need arose for a high temperature adhesive to bond titanium and composite materials on future aircraft. To satisfy this need, LARC-13, a thermoset addition polyimide adhesive, was developed by NASA for bonding an experimental graphite/polyimide composite aircraft wing panel. LARC-13 is illustrated by the formula: ##STR1## Although LARC-13 has demonstrated excellent properties for this specific application, its use is limited because of its brittle nature. A definite need exists for a method to toughen LARC-13 and thereby broaden its utility as an adhesive for bonding structural materials.
Since the 1940s, the incorporation of rubber particles into rigid "low-temperature" polymers such as polystyrene or polyvinylchloride has resulted in greatly successful commercial products. Several-fold increases in the properties of impact strength, elongation, and fracture toughness have been accomplished by the addition of rubbers to rigid polymer matrices. Unfortunately, however, these gains in toughness are almost always associated with a sacrifice in tensile strength, modulus, and other thermomechanical properties.
More recently, worthwhile improvements in the toughness of thermoset epoxy resins have been accomplished by adding liquid CTBN (carboxyl-terminated butadiene/acrylonitrile) as described in Polymer Engineering Science, Vol. 13, p. 29 (1973) by J. N. Sultan and F. J. McGarry. Also, the fracture energy of the DGEBA (diglycidyl ether of bisphenol A) epoxy has been improved by a factor of 15 by adding CTBN copolymers, described in Modern Plastics, Vol. 49, p. 110 (1970) by E. H. Rowe, A. R. Siebert and R. D. Drake.
Problems remain, however, in the toughening of higher temperature polymers such as polyimides. Commercially available rubbers are generally nonaromatic and do not have the thermal stability required for high temperature aircraft/spacecraft applications. Another problem that must be overcome in toughening high temperature polymers with rubbers is the difficulty in meeting the compatibility requirements between the resin and rubber. Finding a suitable solvent for both the matrix polymer and rubber can be difficult. If the rubber is not soluble in the matrix solvent, the rubber will separate out or merely float to the top when the two are mixed. Good rubber/polymer matrix compatibility is almost a necessity in preparing a material that will be easily processable. Further difficulties can arise upon curing the rubber-containing resin, during which a lesser degree of compatibility is desired. In order to achieve maximum toughness in the fully cured state, the rubber must form a fine, evenly dispersed microphase in the polymer matrix. Meeting all of the above requirements can be extremely taxing.
By use of the present invention, a high temperature rubber-toughened addition polyimide adhesive can be produced without a serious sacrifice in thermomechanical properties. The initially desired compatibility between the resin and rubber phases can be achieved by chemically reacting the rubber into the prepolymer backbone. It is anticipated that the toughened addition-type adhesive of the present invention will prove useful for bonding metals and composites on aircraft or spacecraft where resistance to peel forces and adhesive fracture are major criteria.
It is therefore an object of the present invention to provide a novel method for preparing a high temperature addition-type polyimide adhesive containing an aromatic amine-terminated elastomer.
Another object of the present invention is to provide a process for toughening addition polyimide adhesives useful for high temperature structural bonding.
A further object of the present invention is to provide a process for improving the peel strength of high temperature addition polyimide adhesives.
Another object of the present invention is to provide a process for improving the resistance to adhesive fracture of high temperature addition polyimide adhesives.
An additional object of the present invention is to provide a controlled molecular weight prepolymer adhesive composition and process for obtaining same.